The production of transparent objects in various shapes plays an important role in view of the many applications of such objects. In some fields, many transparent products are obtained converting a semifinished glass product.
A high quality of semifinished glass products is often desired in order to achieve particular dimensional characteristics. Among these, a very relevant parameter, is the thickness of its walls.
Among the products whose thickness has to be controlled precisely there are glass tubes, which are produced continuously on special automatic production lines.
The determination of the thickness of glass tubes is used, furthermore, to obtain an indirect measure of the inner diameter of the tube, which is otherwise difficult to be measured on the production lines, for the difficulty of putting sensor elements of measuring instruments within the tube.
Among the many articles obtained starting from a glass tube, for example, the following can be cited: containers used in pharmaceutics such as vials, small bottles, carpules, syringes, as well as devices for laboratories such as graduated cylinders, pipets, burettes, refrigerant tubes, etc., adopted in chemical laboratories, and also sleeves for tubes used in the solar collectors.
In the industrial field, glass tubes have to comply with particular quality adjustments and predetermined dimensional characteristics for allowing their use on the converting production lines.
One of the defects that can affect the quality of the glass tube is a disuniformity of its thickness i.e. a non-coaxiality between the outer surface and the inner surface, with negative consequences on the workability of the final product.
The most common, practical, precise and flexible process for making glass tubes, with diameters and thicknesses that cover most of the market needs, provides a step of hot shaping that is carried out downstream of an oven in an automatic production system which can develops either at an angle or vertically.
More precisely, the production system which develops at an angle consists of a rotating hollow mandrel on which a “casting beak” casts a continuous flow of glass. Through the recess air is insufflated, in such a way that the free end of the mandrel generates a tube.
Alternatively, the machine which develops vertically consists of an orifice, made directly on the bottom of the inlet channel of the molten glass. In the orifice a “bell” is suspended, normally not rotating, which blocks partially the orifice and leaves a circular slit through which the molten glass passes. This way, the molten glass is cast through the orifice sliding on the outer side of the bell, and is still plastic in order to form the tube.
In both cases the tube is then “pulled” by a special machine arranged at a certain distance (where the temperature of the tube is enough low), at the beginning of a conveyor comprising substantially horizontal rollers. The size of the tube is controlled automatically and continuously acting on the flow rate and pressure of air, and on the pulling speed (oblique production line), or acting on the temperature of the zone of the orifice (vertical production line), responsive to the diameter and of the determined thickness of the tube.
It has been found that the defect of non-concentricity has more relevance for the vertical production line, where it is possible to correct it under small movements of the bell. The problem is that the measurement of concentricity is normally made “off-line” i.e. when the tube already reached the end of the line and has been cut, thus obtaining a measurement in absolute value, but loosing the information on the direction of the non-concentricity. To obtain this information it is necessary to provide on the measured tube a sign at the angular reference of the orifice. Typically the surface of the tube is “marked” with a pen of a suitable material in a fixed position, nearest to the orifice, so that non-concentricity is referred to the reference system of the orifice, and then the correction of the position of the bell is determined.
This procedure causes various drawbacks, among which the main is the need to deposit extraneous material on the glass tube, with the risk of leaving residues on the rollers and on the many conveying devices, causing also high risk of contamination of the product, which is typically destined to the market of pharmaceuticals. For avoiding this risk, after the step of marking, all the marked tube glass product is rejected and discarded, and since the adjustment by marking can last different tenths of minutes, a huge loss of material, efficiency, as well as costs, and energy waste occur.
Moreover the step necessarily affects the thermo-mechanical balances on which the process is based, with the effect measuring the process in a transient in which the adjustments are made, instead of a steady status.
Finally owing to the high temperature, the measurement is critical, further limiting the precision and the frequency with which it can be carried out.
It is therefore desirable to measure shape defects of the glass tube immediately downstream of the formation step, possibly by means of a “contactless” measurement, in order to intervene in the formation step just upstream of the measurement step in real time, and correcting the defect, limiting to the minimum the waste of product, and especially the risks of bad quality of the product. In fact, by making a precise measurement of the thickness of the tube, it is possible to adjust the step of tube shaping to avoid thickness defects.
Among the known thickness measurement systems of transparent object the interferometric one is known that provides sending a light beam on the transparent object and collecting the reflected radiation. More precisely, the reflection is exploited taking into account that both the outer surface and the inner surface produce a reflected light component, even if of minimum intensity (for the glass each reflected light component is about 4% of the incident radiation on the interface). This way, the reflected light beam is given by the overlapping of two reflected radiation having amplitude of the same order of quantity and, said currents being phase-shifted from each other, corresponding to a longer path made by the reflected radiation through the inner surface with respect to that reflected by the external surface. Such overlapping causes phenomena of interference, which can be examined to determine the difference of path, and then the thickness.
This type of measurement is common for computing the thickness of a film thin whose thickness is up to some micron, but for larger thicknesses technologies of collecting and controlling the signal are required, of much higher precision, and a more advanced and expensive apparatus are needed.
For this reason, it is necessary to collect the reflected radiation in the in the most effective way possible, in order to obtain a signal for computing the thickness of glass objects, about two or three orders of magnitude thicker with respect to the film thin.
Further problems, concerning the automatic production lines of glass tubes, are due to the fact that the glass tube is not perfectly still, because it runs quickly, and then it is subject to movements and vibrations, such that the reflected radiation has a variable direction, and the lower is the diameter of the tube the higher is the flexibility, causing large difficulty in the collecting it reliably for spectroscopic analysis.
It is therefore desirable that the collected reflected radiation is minimally influenced by a possible vibration or small movement of the glass tube, or more normally of the glass object whose thickness has to be determined, and to collect a sufficient output signal for carrying out a spectroscopic analysis.
Yet another consideration is that, if applied to a glass tube or other object with curved surface that is invested by a light beam, in order to determine the thickness by spectroscopic analysis, the existing measuring systems can provide the sought thickness data only from the side of the wall invested by the beam, i.e. that receives the incident radiation and recovers a reflected radiation that can be collected at the same side from which the incident beam comes. For determining the thickness of a sufficient number of points, it is then necessary to carry out more measurements and then prearranging a number of instruments higher for increasing the thickness measurement points. This causes a subsequent increasing of necessary costs and of resources.
It is therefore desirable to determine the thickness and especially its variation in a number appropriate of points, in order to determine any defects of shape, i.e. non-coaxiality between the inner surface and the external surface, without an increase of the costs.